A digital hardware implementation of Tangent Sigmoid (Tan-Sig) activation function based on sigma-delta modulated bit-streams is proposed.Through the change of feedback coefficient in the framework of traditional sigma-delta modulator

a new limiting amplifier modulator (LAM) is fabricated

and the function approximation to Tan-Sig was achieved by the combination of several LAMs with different coefficients.Meanwhile

the bit-stream neurons with Tan-Sig activation function and the whole feed-forward artificial neural networks are implemented on field programmable gate array (FPGA).And the 3-D space classification problem in Cartesian coordinate system was solved by the neural networks presented.